1. Field of the Invention
The present invention is related to the field of communication systems and more particularly to a method and circuit for mitigating static and/or dynamic DC offset or baseline wander at the transmitter, receiver, and/or in the channel.
2. History of the Related Art
Baseline wander is a well-known phenomenon observed in a wide variety of applications. Such applications may include, but are not limited to, any system that contains low frequency, random data signals, or uses transformer coupling, AC coupling, DC notching, and all systems suffering from DC drift and baseline wander at the transmitter, receiver, or in the communication channel itself. Applications that may experience such baseline wander may include wire line modem systems, digital subscriber line (DSI) and cable communication systems, Ethernet, radar, and television receivers. Typically, the principle methods of trying to compensate or eliminate baseline drift in a communication system include methods in which the received signal is observed over time to obtain an estimate of the DC offset or the DC droop by integrating the signal over time. This approach typically requires long term signal averaging that slows the response time and makes this type of approach difficult to implement in a system where the baseline wander changes rapidly. Only if the DC droop is significantly larger than the signal and is slowly changing over time would such an approach be appropriate. Otherwise, the droop estimate will be masked by the running sum of the inherently locally imbalanced signal samples. In other words, by integrating the signal over time, instantaneous signal information is lost and the sensitivity of the system to address rapidly changing baseline wander is limited. In another approach, an inverse filter is designed and used to directly cancel the effect of the channel and/or receiver component that causes the DC error. Unfortunately, this method requires knowledge of the drift source, its characteristics, and its invertability. Unfortunately, this information is not always readily available. For example, AC coupling transformers that are used in twisted pair applications typically have a zero at s.sub.z =0 and hence their inversion requires a poll at s.sub.p =0, which results in an unstable system. A third approach to addressing DC wander involves replicating the DC wander at the receiver by passing the detected data symbols through a filter whose frequency response is similar to that of the channel/receiver causing the DC error. This approach requires accurate knowledge of the drift causing channel and/or receiver component as well as reliable detection of transmitted symbols. Therefore, it would be highly desirable to implement a communication system incorporating the ability to cancel time varying DC offset that overcomes the described limitations of conventional approaches.